Tuning Into the Hum of Spinning Stars express
After a supernova explosion, a neutron star neutron star remains: a city-sized ball of unimaginably dense matter, spinning hundreds of times per second. Any tiny bump on its surface would make it wobble like an off-balance spinning top, humming faintly through the fabric of space. That hum is a gravitational wave gravitational wave — a ripple in spacetime spacetime predicted by Albert Einstein.
Astronomers tuned the LIGO detectors, created by scientists like Rainer Weiss, to 34 of these spinning stars, listening for their hum. After the most extensive search yet, they heard nothing.
The quiet is revelatory: the Crab star pours less than 0.04% of its rotational energy into gravitational waves. Extrapolate that smoothness across the cosmos, and these stars are rounder than any object we know. If we ever catch their faint whisper, it will open a window into the densest matter in the universe.
Original abstract
as written by the authors arXiv:2603.25938
Rotating non-axisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). Such CWs can, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which an efficient and sensitive matched-filter search can be applied. Narrowband searches are designed to be robust to mismatches between the electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where the CW emission is assumed to be phase-locked to the EM one. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO--Virgo--KAGRA observing run. This is the largest number of pulsars so far targeted for narrowband searches in the advanced detector era. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency -- frequency time derivative ($$f$$ -- $$\dot{f}$$) space. Here, we also explore a range in the second time derivative of the frequency $$\ddot{f}$$ around the value indicated by EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar, using simulated signals added in real data. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude is $$\lesssim 2\%$$ of its spin-down limit, corresponding to less than $$0.04\%$$ of the spin-down power being radiated in the CW channel.
A teaspoon of neutron star stuff weighs about 10 billion tons, like packing Mount Everest into a sugar cube.